Based on news reports, it would be difficult to overestimate how many bugs Mac OS X 10.5.5 managed to fix. Macworld reported that the update addressed “33 issues with the operating system.” Other sites suggested that 10.5.5 patched “nearly 70 bugs.”. According to Ars Technica, the update provided “tons of fixes.”

Depending on your viewpoint, it’s either impressive how much Apple fixed or how much still needed fixing, considering this is the fifth maintenance update to OS X 10.5.

You might suppose that, given all of the hyperbole, after you updated to Mac OS X 10.5.5, you would easily notice at least a few of these improvements. “Not so!” claim many, if not most, Mac OS X users. These users report seeing not even one difference between 10.5.4 and 10.5.5.

How can this be? What’s the explanation for this apparent paradox? Quite simple, really.

Many fixes only affect a subset of Mac users: For example, Apple’s Knowledge Base article about 10.5.5 reports that the update “addresses an issue in which some Macs could unexpectedly power on at the same time each day.” Apple has also posted a separate article on this symptom, explaining that the problem was due to an apparently corrupt com.apple.AutoWake.plist file, located in /Library/Preferences/SystemConfiguration.

Apple doesn’t explain exactly what may go wrong with this file or what causes it to go wrong, or why it no longer goes wrong in 10.5.5. Regardless, chances are good you never had this problem anyway, as it only affects “some Macs.” If you’ve never had the symptom, you won’t notice any change after updating to 10.5.5.

By the way, if you haven’t updated to 10.5.5, deleting the com.apple.AutoWake.plist file should fix the problem.

You don’t use an affected application: It’s hard to notice a bug fix if you never use the application that’s been fixed. For example, Apple reported that Mac OS X 10.5.5 “resolves an issue in which the ‘Refresh All’ option may be dimmed (‘grayed out’) in the contextual menu for certain calendars.” Obviously, if you don’t use iCal, this remedy is irrelevant to you. Even if you do use iCal, echoing my first point, you may not have one of the affected “certain” calendars.

Some fixes take place behind-the-scenes: And if they’re behind the scenes, you’re unlikely to notice them. Thus, Apple reported that “Time Machine can now back up iPhone backups that are on your Mac, as well as other items in (~/Library/Application Support).” Never mind that the remedy is only relevant if you use Time Machine. I suspect that even most Time Machine users were oblivious to whether or not Time Machine backed up this folder in 10.5.4. So they would be hard-pressed to notice any change in 10.5.5.

Security fixes for potential problems are under the radar: You’re not likely to feel effect of security fixes, as they generally protect against potential attacks that no one is ever going to try on your computer anyway.

In other words, despite all the improvements in Mac OS X 10.5.5, it would probably take less the fingers on one hand to count the number of fixes you’ll ever notice. Overall, this a good thing. By this point in Leopard’s life cycle, we shouldn’t be seeing the eradication of big, obvious and pervasive bugs. They should have already been fixed in earlier updates. Happily, this appears to be the case.

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